Japanese Patent No. 4755572 discloses a system that includes a heat accumulator for storing/circulating engine cooling water in an engine cooling water circulation circuit that connects an engine and a heater core of an air-conditioning unit. An engine-side valve is disposed in a communication portion between an outlet/inlet on the engine side and an inlet on the heat accumulator side in the engine cooling water circulation circuit. Further, a heater-side valve is disposed in a communication portion between an outlet/inlet on the heater core side and an outlet on the heat accumulator side.
In the system disclosed in Japanese Patent No. 4755572, which includes the above-described engine cooling water circulation circuit, four modes, namely, a “heat accumulation mode”, a “heat accumulation maintaining mode”, an “engine immediate warming mode” and a “in-cabin immediate warming mode”, can be selected by switching the aforementioned valves. Specifically, when the engine is stopped, the “heat accumulation maintaining mode” is selected that blocks the outlet/inlet on the heat accumulator side by placing both of the engine-side valve and the heater-side valve at a closed position so that high-temperature engine cooling water is thermally insulated and stored inside the heat accumulator.